Electromagnetic interference has been defined as undesired conducted or radiated electrical disturbances from an electrical or electronic apparatus, including transients, which can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur anywhere in the electromagnetic spectrum. Radio frequency interference is often used interchangeably with electromagnetic interference, although it is more properly restricted to the radio frequency portion of the electromagnetic spectrum, usually defined as between 10 kilohertz (kHz) and 100 gigahertz (GHz).
A shield is defined as a metallic or otherwise electrically conductive barrier inserted between a source of EMI/RFI and a desired area of protection. Such a shield may be provided to prevent electromagnetic energy from radiating from a source. Additionally, such a shield may prevent external electromagnetic energy from entering the shielded system. As a practical matter, such shields normally take the form of an electrically conductive cover or enclosure, which is electrically in contact with either: the ground trace on a PCB (printed circuit board); or another mating cover (with the PCB completely surrounded by and grounded to, the two cover halves). Any unwanted EMI/RFI energy is thereby dissipated harmlessly to ground. Most such enclosures or shields are open on at least one side, or are provided with access panels, doors and/or removable lids. Thus, the shields have at least one removable interface, which is typically used to allow the user to gain access to the electronic circuitry underneath.
Gaps that form at the removable interface (typically between the shield and the PCB or between the two shield halves) provide an undesired opportunity for electromagnetic energy to leak into the shielded system. The gaps also can interfere with electrical current running along the surfaces of the shield, as well. For example, if a gap is encountered, the impedance of the gap is such that electromagnetic energy may radiate from an opposing side of the gap, much like a slot antenna. A device used to fill such gaps is known as a gasket. As used herein, a "gasket" is defined as a device that fills a gap in a shielded system at a removable interface, such as between a shield and a PCB, or between two shields, with a PCB enclosed therein, for example.
Various configurations of gaskets have been developed over the years to conformably fill these gaps and to effect the least possible disturbance of the ground conduction currents. Each seeks to establish an electrically conductive path across the gaps--the higher the conductivity, the better. However, there are inevitable compromises between: the ability of the gasket to smoothly and thoroughly engage and conform to the surface of the shield and its mating surface; the conductive capacity of the gasket; the gasket's resilience; the gasket's softness; the ease of mounting the gasket; and the cost of manufacturing and installing the gasket.
Presently, many electronic devices, such as but not limited to: pocket pagers, cellular phones, laptop computers and wireless local area networks (LANs), are constructed using metallized plastic injection-molded shields which are not manufactured to exact tolerances. Therefore, the aforementioned gaps in these systems can be quite significant. Typically in such devices, mating shield members incorporate a snap-together method of closure, or in other instances, they utilize a limited number of light gauge screws. Accordingly, most electronic devices having metallized plastic injected molded housings, cannot deliver substantial closure force to assemble a housing/PCB system together. Any EMI gasket that is incorporated into such electronic devices must therefore be deformable under a low compression force. If the gasket material is too hard, it will not conform to the irregular surfaces between the housing and cover, and therefore, the gaps will not be completely sealed. These gaps can sometimes be so large that slots are created, from which electromagnetic energy can escape. Additionally, a gasket material that is difficult to compress can result in bending or bowing of the shield and PCB, which can result in many other mechanical problems, such as stress relaxation in the plastic shield, problems with solder joints cracking on the bowed PCB, etc. Therefore, it is imperative that the gasket material is soft enough to conform to irregular surfaces when: the fastener spacing is large, the stiffness of the shield(s) and/or PCB is very low, and when only a small amount of force is available from the snaps or screw fasteners.
Conventionally, conductive particle-filled silicone elastomers have been utilized as conductive gaskets, to reduce EMI and RFI. However, such materials tend to be relatively hard (typically Shore A hardness of 45 or greater). Because of their hardness, these conductive elastomers are not well suited for use as a gasket in a device having a shield that is assembled with a substantially low closure force. Additionally, these conductive elastomers are difficult to manipulate when formed or die-cut into complicated gasket patterns, with narrow gasket sections. Since cellular phone EMI shields, for example, utilize lightweight, flexible and extremely small plastic parts, an alternative to hard, conductive elastomers is desired in the industry.
Other EMI/RFI shielding gaskets have been proposed which incorporate a conductive fabric or mesh which surrounds a soft, conformable foam material. Examples of such gaskets are disclosed in U.S. Pat. Nos. 5,028,739; 5,115,104; 4,857,668; 5,045,635; 5,105,056; 5,202,526; and 5,294,270. Although the gaskets disclosed in the foregoing U.S. patents may be deformable under a low compression force, these gaskets do not have continuous conductivity throughout the material. These gaskets are typically constructed of an inner, non-conductive foam support material that is wrapped with a metallized, conductive fabric, rendering only the outer fabric surface conductive. Continuous conductivity, throughout the entire cross-section of the material, is required for proper EMI shielding. Because they are not continuously conductive, these gasket materials cannot therefore be die-cut into arbitrary, complex shapes to function as an EMI/RFI gasket. The ability to die-cut a material (into such a complex shape) is especially important for multi-cavity enclosures with narrow walls, typically less than 3 mm in width.
U.S. Pat. No. 4,931,479 describes a conductive polyurethane foam material, which is rendered conductive by filling a base material with conductive particles and then "foaming" the base material. One suggested conductive particle is carbon. Although such a conductive polyurethane foam material may be effective for use in certain applications (particularly ESD--electrostatic discharge applications), it is not sufficiently conductive for use as an EMI/RFI shielding gasket in the frequency range from about 10 MHz to about 26 GHz. Additional suggested conductive fillers are metallic particles such as silver, nickel, copper, nickel-plated graphite, silver-plated glass, silver-plated nickel, and silver-plated copper, for example. However, for such a shielding gasket to be effective in the frequency range from about 10 MHz to about 26 GHz, the base material would probably have to be loaded with such a high density of metallic conductive particles that the gasket would probably exhibit poor mechanical properties. Such mechanical properties might be softness, recoverability, tensile strength, etc.
U.S. Pat. No. 3,666,526 discloses an electrically conductive metallized porous foam. In theory, such a material could potentially be used as a conformable EMI gasket, however, there are several practical problems with this. First, such metal foams are typically heavily coated with metal and subsequently sintered, removing the base polymer foam structure. This makes the structure more difficult to compress, as well as reduces the possibility for the gasket material to recover to its original height, once compressed. Dimensional recoverability is a critical feature for an EMI gasket to maintain, since this can also affect electrical performance over time. Also, particulation of the metal coating because of its coating thickness, may be an issue, with this type of solution.
An attempt to overcome the problems of recoverability and particulation was disclosed in patent number U.S. Pat. No. 4,576,859, for example. The invention is described as a metallized foam material with a conductive outer coating of rubber and/or plastic surrounding an inner, metal coating surface on a foam substrate. This conductive outer coating was provided to aid in resilience (recoverability) and also to prevent the metal coating from peeling off of the skeletal structure of the foam, thereby preventing particulation. This extra coating process adds both complexity and cost to the final article that detracts from its use as an EMI gasket. Also, since most particle-filled conductive coatings are not typically as conductive as the metal coating itself, this added step may also reduce the gasket's electrical performance. Also, U.S. Pat. No. 4,576,859 describes a material that has a relatively low pore density, between 10 and 40 cells per linear 25.4 mm. This makes the product extremely difficult to handle and install, especially when die-cut or formed into narrow, complicated shapes.
An EMI or RFI shielding gasket material which is substantially deformable under low pressure, dimensionally recoverable, able to be handled and installed when die-cut or formed into narrow-walled, complicated shapes, and simultaneously, continuously electrically conductive throughout the structure of the material, is desirable. With sufficient electrical conductivity, such a material would be able to provide excellent EMI shielding in a frequency range from about 10 MHz to about 26 GHz, while being compatible with the use of lightweight plastic shields, snap features, and thin PCB's. Also desirable is such a gasket that is adapted for rapid and efficient application to a PCB or an EMI/RFI shield.